Detections of planets by NASA's Kepler space telescope has led to recent estimates that roughly 22 percent of sun-like stars might host a rocky Earth-size planet within their habitable zones. With some 100 billion stars in the Milky Way, this hinted there could be as many as 22 billion Earth-like planets in the galaxy.

However, this number depends on defining the habitable zone of a sun-like star as ranging from one-half to two astronomical units (AU) around it, where one AU is the average distance between Earth and the sun, some 93 million miles (150 million kilometers).

The new alien planet atmosphere analysis, released by a team led by astrophysicist Jérémy Leconte, of the Pierre Simon Laplace Institute in Paris, in the journal Nature, narrows back the inner edge of habitable zones around sun-like stars to about 0.95 AU from those stars, roughly 90 million miles (145 million kilometers).

This could nearly halve the estimated number of Earth-like planets in the Milky Way.

New, More Complex Model

Past models of temperatures on alien worlds essentially treated such planets as simple dots, one-dimensional objects that averaged the amount of heat they reflected or absorbed from their stars. The new research instead uses a 3-D climate model, which can account for details such as the way air flows.

"We can start to treat exoplanets as real three-dimensional planets, where complex processes like cloud formation can occur," said Leconte.

One factor the study model analyzed is water vapor, which traps heat. If a world is too close to its star, too much water on its surface can vaporize, heating that planet enough to eventually cause all its water to vaporize, rendering its surface uninhabitable to life as we know it.

Researchers suspect this "runaway greenhouse" effect is what happened to Venus in our solar system.

Until now, scientists thought clouds of water vapor helped cool planets by reflecting heat back into space. The new model reveals that some clouds instead might trap heat and help destabilize climate on alien worlds. Although clouds near the surface of planets do reflect heat back into space, clouds at high altitudes are colder and so absorb some of this heat, allowing less of it to escape, Leconte said.

Stabilizing Climate

Although these findings regarding clouds suggest it might be much easier to send planets hurtling toward runaway greenhouse scenarios than previously thought, the new model showed that there are other factors that help stabilize climate.

For instance, an atmosphere moves warm, moist air from tropical regions to colder polar ones. "These regions are very important for stabilizing a planet's climate, keeping it from a runaway greenhouse," Leconte said.

In addition, the new model reveals the "moist greenhouse effect," in which the host star's light is thought to cook away water vapor in the upper atmosphere, may be much less of a concern for the habitability of planets than previously thought.

"We found the upper atmosphere of planets gets much colder than it was thought to get," Leconte said. "This means any water vapor would [turn to] rain or snow it before it gets to very high altitudes where it can get broken down."

Where to Look for Habitable Planets

The new 0.95 AU estimate for the inner edge of habitable zones is actually very similar to some other estimates from simpler models of alien climates, acknowledge the researchers.

"At first it was a little disappointing not finding a bigger difference," Leconte said. "The important thing is we now are finally beginning to understand how the climates of real planets might behave."

Planetary scientist Ravi Kopparapu at Pennsylvania State University, who did not take part in this research, agreed it was likely that the inner edge of habitable zones for Earth-size planets in sun-like systems lies beyond 0.5 AU. "Venus, which is completely desiccated, is at 0.72 AU," Kopparapu said. "That is telling us the inner edge of the [habitable zone] probably lies beyond 0.72 AU."

Planetary scientist James Kasting, who is also at Pennsylvania State University and was not involved in this study, noted that if the habitable zone is narrow, "then many stars must be searched to find an Earth-like planet, and the telescope must be correspondingly large."

Future Directions

Future research by the team will explore if 3-D models alter estimates of where the outer edges of habitable zones lie. Current estimates for the outer edge of habitable zones for Earth-mass planets around sun-like stars range from 1.7 to 2 AU. "The way air circulates in a 3-D model could keep water from freezing at greater distances from stars than before thought," Leconte said.

Scientists can also investigate what climates are like in systems unlike that of Earth and the sun. For instance, planets around smaller stars likely get tidally locked, always keeping the same side facing their stars.

"This means that, like the moon around the Earth, they always present the same day side to the star and have a permanent night side," Leconte said. "This will profoundly change both the atmospheric circulation and the location of clouds."